1. Field of the Invention
This invention relates to earth boring bits used in the oil, gas, and mining industry and in particular to rolling cone drill bits with lubricant systems sealed by volume compensating rigid face seals.
Modern sealed and lubricated earth boring drill bits (also called rock bits) have very limited space available for their dynamic lubricant seal. Furthermore, rigid bearings are not practical in these bits and axial and radial cutter movements with respect to the bearing shaft occur frequently during operation. Most sealed rock bits are designed to have some of this bearing play, resulting in some amount of axial displacement of the rolling cutter onward and offward the cantilevered bearing shaft during operation. An early, commercially successful Belleville type bearing seal for rock bits is shown in U.S. Pat. No. 3,137,508. This patent describes how movements of the rolling cutter cause volume changes in the lubricant which can generate large pressure differentials at the dynamic seal in the cutter. Later, when elastomeric packing ring type seals were developed for rock bits in the late 1960's, they were more tolerant of the pressure differentials and quickly replaced the Belleville spring seal in premium rock bits.
In spite of the limited space and extreme environment, there are many potential benefits in providing rigid face seals in rock bits. Therefore, there are many rigid face seal designs for rock bits which strive to minimize the effects of these pressure fluctuations of the lubricant near the seal caused by the bearing play. In U.S. Pat. No. 5,040,624 a large channel connects between the seal cavity and the pressure balancing diaphragm to minimize these pressure differentials. In another design, shown in U.S. Pat. No. 4,753,304, the geometry of the seal and bearing cavity is arranged such that lubricant volume changes next to the rigid face seal are minimized, thus minimizing pressure differentials caused by bearing play. In addition, there are many face seal designs that allow lubricant to be expelled from the bit in response to these pressure spikes.
Other similar rigid face seals for rock bits are shown in U.S. Pat. Nos. 3,370,895; 3,529,840; 4,176,848; 4,178,045; 4,199,156; 4,249,622; 4,306,727; 4,344,629; 4,359,111; 4,394,020; 4,516,640; 4,613,005; 4,747,604; 4,762,189; 4,822,057; 4,824,123; 4,838,365; and 5,009,519.
A common characteristic of all the above rigid face seal designs is that they cannot displace an effective volume of lubricant near the bearing to limit the pressure fluctuations caused by the bearing play. The above designs either have very limited axial movement and therefore are prevented from sweeping through a volume, or they are arranged in a manner where they do not displace a volume as they move. None of these designs have had widespread commercial success, due in part to this inability to effectively compensate for lubricant volume changes caused by bearing play. These seal designs are known as non-volume compensating type face seals, and the above patents are listed primarily for background information.
One of the above non-compensating face designs, shown in U.S. Pat. No. 4,838,365, is the ancestor of the present invention. Several bits with this particular non-volume compensating face seal were field tested. However, the non-volume compensating metal face seals in these bits did not meet expectations due to sealing face overload and excessive slippage of the shaft ring energizer. It is doubtful that these bits would perform even as well as bits with elastomer seals if run in a severe drilling environment.
In 1985 a volume compensating rigid face seal for rock bits was invented by Burr and granted U.S. Pat. No. 4,516,641. Burr designed a rigid face seal assembly for rock bits which moved axially within a groove in response to local lubricant volume changes near the bearing caused by axial displacement of the cutter. The axial movement of this seal minimized the pressure differentials caused by axial displacement of the cutter. Therefore, as axial and radial cutter movement occurred due to bearing play, there was minimal net lubricant volume change adjacent to the seal within the cutter. This design is known as a volume compensating seal because it is a rigid face seal assembly which moves axially in either direction from an equilibrium position in response to local lubricant volume changes.
There is enough clearance in each end of the seal cavity to accommodate the axial travel of the seal anticipated within the cutter during normal operation. The pressure variations generated during volume compensation are relatively small and are related to the stiffness of the seal's energizers and the effective area swept by the seal. Other rigid face seals for rock bits which could be considered volume compensating are shown in U.S. Pat. Nos. 3,713,707; 4,306,727; 4,466,622; 4,666,001; 4,671,368; 4,753,303; 4,903,786; 4,923,020; 5,295,549; and 5,360,076.
Although many of these seal designs are successful, they are not without problems. One weakness of these prior art volume compensating face seals is unintended rotation of the bearing shaft seal and energizer ring upon the cantilevered bearing shaft. This rotation often leads to destruction of the shaft energizer. Unintended rotation of the seal ring mounted on the bearing shaft is a well documented problem in prior art volume compensating rigid face seals. Means to prevent this rotation are addressed in previously referenced U.S. Pat. Nos. 4,306,727; 4,466,622; and 5,295,549.
A second problem in these prior art rigid face seal designs is that the force on the seal face can vary during operation as the seal assembly moves in response to lubricant volume changes. If the sealing face force were to drop significantly, lubricant could be lost from, or drilling fluid could enter, the bearing cavity, leading to rapid bearing degradation of the bit. Also, a large increase from the initial sealing face load can cause excessive heat generation and adhesive wear at the sealing faces, leading to failure of the seal.
Another problem with all rigid face seals in rock bits is abrasive wear of the sealing faces caused by intrusion of fine abrasives from the drilling fluid. Typically a 0.040 inch to 0.060 inch wide, smooth and flat sealing band is formed upon the sealing faces. The sealing band on these seal faces is placed as closely as possible to the outer periphery of the seal rings to minimize the intrusion of abrasive particles. This slows abrasive wear of the sealing faces, but does not prevent it.
Adhesive wear of the seal faces is caused by asperity contact of the mating seal faces. If the seals are made from materials which resist adhesive wear, the abrasives can still intrude into the edge of the seal face, cause abrasive wear, and slowly cause the sealing band to become ever narrower until there is no flat sealing band left to seal. At this point, abrasive laden drilling fluid may enter the bit and cause bearing failure.
Consequently, a common strategy is to make the seal rings with a material and a geometry so that normal material loss from the sealing faces from adhesive wear will cause the sealing band to widen. As abrasive wear reduces the width of the seal band from the periphery, the adhesive wear causes the seal band to expand toward the inside diameter of the seal ring. If everything is properly designed, the arrangement maintains an equilibrium seal band width. The end result is a sealing band that stays about 0.040 inches to 0.060 inches wide and slowly precesses toward the inside diameter of the seal ring. Variations in sealing face load can profoundly affect this equilibrium and unexpected wear patterns can still lead to premature seal failure.
Finally, the prior art does not address the problem associated with differential pressurization of the lubricant with respect to the drilling fluid. Pressure balancing diaphragms in rock bits are typically used in association with a lubricant pressure relief means. These devices typically allow the lubricant to become differentially pressurized to 100 to 200 PSI greater than the drilling fluid under some types of drilling conditions before they expel lubricant into the drilling fluid to limit the pressure buildup. A volume compensating rigid face seal in a rock bit may be moved within its cavity by this sustained pressure differential leading to the same type of sealing face force variations described earlier. Under extreme conditions the seal assembly could move so far that it contacts the end of the seal groove. When this happens, the seal loses its ability to compensate for lubricant volume changes--possibly causing very rapid seal failure.
The present invention provides a volume compensating rigid face seal which mitigates the above problems. The invention provides a rigid face seal for a rock bit which minimizes the variation in face loads as the seal assembly moves in response to lubricant volume changes. Another feature of the invention is that slippage of the shaft energizer is also minimized. Finally, a bit made in accordance with the present invention has a volume compensating rigid face seal which better tolerates differential pressurization of the lubrication with respect to the drilling fluid.